On 3/26/20 7:52 PM, Bart Van Assche wrote:

On 2020-03-26 17:19, Dongli Zhang wrote:

quoted

I think the issue is because of line 2827, that is, the q->nr_hw_queues is
updated too earlier. It is still possible the init would fail later.

2809 static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
2810                                                 struct request_queue *q)
2811 {
2812         int i, j, end;
2813         struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx;
2814
2815         if (q->nr_hw_queues < set->nr_hw_queues) {
2816                 struct blk_mq_hw_ctx **new_hctxs;
2817
2818                 new_hctxs = kcalloc_node(set->nr_hw_queues,
2819                                        sizeof(*new_hctxs), GFP_KERNEL,
2820                                        set->numa_node);
2821                 if (!new_hctxs)
2822                         return;
2823                 if (hctxs)
2824                         memcpy(new_hctxs, hctxs, q->nr_hw_queues *
2825                                sizeof(*hctxs));
2826                 q->queue_hw_ctx = new_hctxs;
2827                 q->nr_hw_queues = set->nr_hw_queues;
2828                 kfree(hctxs);
2829                 hctxs = new_hctxs;
2830         }

Which kernel tree does this syzbot report refer to? Commit
d0930bb8f46b ("blk-mq: Fix a recently introduced regression in
blk_mq_realloc_hw_ctxs()") in Jens' tree removed line 2827 shown above.

Thank you very much for sharing this. The below is in Jens' tree for 5.7.

commit d0930bb8f46b8fb4a7d429c0bf1c91b3ed00a7cf
Author: Bart Van Assche [off-list ref]
Date:   Mon Mar 9 21:26:18 2020 -0700

    blk-mq: Fix a recently introduced regression in blk_mq_realloc_hw_ctxs()

    q->nr_hw_queues must only be updated once it is known that
    blk_mq_realloc_hw_ctxs() has succeeded. Otherwise it can happen that
    reallocation fails and that q->nr_hw_queues is larger than the number of
    allocated hardware queues. This patch fixes the following crash if
    increasing the number of hardware queues fails:

    BUG: KASAN: null-ptr-deref in blk_mq_map_swqueue+0x775/0x810
    Write of size 8 at addr 0000000000000118 by task check/977

    CPU: 3 PID: 977 Comm: check Not tainted 5.6.0-rc1-dbg+ #8
    Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
    Call Trace:
     dump_stack+0xa5/0xe6
     __kasan_report.cold+0x65/0x99
     kasan_report+0x16/0x20
     check_memory_region+0x140/0x1b0
     memset+0x28/0x40
     blk_mq_map_swqueue+0x775/0x810
     blk_mq_update_nr_hw_queues+0x468/0x710
     nullb_device_submit_queues_store+0xf7/0x1a0 [null_blk]
     configfs_write_file+0x1c4/0x250 [configfs]
     __vfs_write+0x4c/0x90
     vfs_write+0x145/0x2c0
     ksys_write+0xd7/0x180
     __x64_sys_write+0x47/0x50
     do_syscall_64+0x6f/0x2f0
     entry_SYSCALL_64_after_hwframe+0x49/0xbe

    Fixes: ac0d6b926e74 ("block: Reduce the amount of memory required per
request queue")
    Signed-off-by: Bart Van Assche [off-list ref]
    Reviewed-by: Ming Lei [off-list ref]
    Cc: Keith Busch [off-list ref]
    Cc: Johannes Thumshirn [off-list ref]
    Cc: Hannes Reinecke [off-list ref]
    Cc: Christoph Hellwig [off-list ref]
    Signed-off-by: Jens Axboe [off-list ref]

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d4bd9b961726..37ff8dfb8ab9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -2824,7 +2824,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
                        memcpy(new_hctxs, hctxs, q->nr_hw_queues *
                               sizeof(*hctxs));
                q->queue_hw_ctx = new_hctxs;
-               q->nr_hw_queues = set->nr_hw_queues;
                kfree(hctxs);
                hctxs = new_hctxs;
        }

That should be the reason why "init_hctx() fault injection" was introduced.

Thank you very much!

Dongli Zhang